Ebullition cooling device for heat generating component

ABSTRACT

The invention provides an ebullition cooling device  1  for a heat generating component B which device comprises a boiling unit  2  for boiling a refrigerant A contained therein with the heat generated by the heat generating component B as attached to an outer surface of the unit, a condensing unit  3  disposed above the boiling unit  2  for condensing a refrigerant vapor A 1  flowing thereinto from the boiling unit  2  by heat exchange with an external fluid C, and a communication pipe  4  interconnecting the units  2, 3  and having a refrigerant vapor channel  41  and a refrigerant condensate channel  42  therein. The cooling device  1  can be designed with greater freedom, and is therefore fully useful for electronic devices which are compacted or higher in complexity, smaller in the amount of refrigerant to be enclosed therein and outstanding in heat dissipating performance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U.S.C. §111(a)claiming the benefit pursuant to 35 U.S.C. §119(e)(1) of the filing dateof Provisional Application No. 60/356,121 filed Feb. 14, 2002 pursuantto 35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to ebullition cooling devices for use incooling heat generating components, such as diodes and transistors,incorporated in electronic devices or the like.

BACKGROUND ART

Ebullition cooling devices are adapted to cool heat generatingcomponents by heat transport due to the boiling and condensation ofrefrigerant, and are used in various electronic devices for cooling heatgenerating components, such as diodes and transistors, incorporated inthe device.

Refrigerants of low boiling point for use in ebullition cooling devicesare generally fluorocarbons and like expensive refrigerants. Preferably,therefore, ebullition cooling devices have such a construction that theamount of refrigerant to be enclosed therein can be smaller to thegreatest possible extent.

Ebullition cooling devices already known for use with heat generatingcomponents include the device disclosed, for example, in JP-A No.8-204075(1996). The disclosed device comprises a boiling unit in theform of a hollow planar plate having a multiplicity of hollow channelsin its interior and a side wall outer surface with a heat generatingcomponent attached thereto, and a condensing unit in the form of a heatexchanger having plate fins. The hollow planar plate constituting theboiling unit is connected directly to the heat exchanger constitutingthe condensing unit so as to hold the hollow channels of the former incommunication with a header tank of the latter.

The cooling device described above has the boiling unit provided by ahollow planar plate, so that the amount of refrigerant enclosed need notbe very great, whereas the arrangement wherein the hollow planar plateis connected directly to the heat exchanger results in a low degree offreedom in designing the device. Accordingly, the device is not fullyuseful for electronic devices which are compacted or higher incomplexity and can not always be installed within such electronicdevices.

When the heat receiving position (e.g., the location where a heatgenerating component is installed) of an electronic device is a largedistance away from the heat dissipating position thereof (e.g., thelocation where a vent is provided), the ebullition cooling devicedescribed must be made usable in the electronic device, for example, bygiving an increased size to the hollow planar plate. This entails acorresponding increase in the amount of refrigerant enclosed.

Further with the ebullition cooling device described, the heatgenerating component is attached to the side wall outer surface of thehollow planar plate which has a relatively small thickness, so thatincreased contact thermal resistance between the plate and the componentdue to insufficient rigidity of the side wall is likely to result inlower heat dissipating performance.

The above device further has the problem that the refrigerant vaporflowing upward from the boiling unit toward the condensing unit and therefrigerant condensate flowing down from the condensing unit toward theboiling unit interfere with each other at the connection between the twounits, i.e., the problem of so-called flooding. This flooding phenomenondegrades the spontaneous circulation of the refrigerant, reducing themaximum amount of heat transport and entailing lower heat dissipatingperformance, and therefore needs to be prevent to the greatest possibleextent.

An object of the present invention is to provide an ebullition coolingdevice for a heat generating component which device can be designed withincreased freedom so as to be fully useful in compacted or complexelectronic devices or the like and is reduced in the amount ofrefrigerant to be enclosed therein and outstanding in heat dissipatingperformance.

DISCLOSURE OF THE INVENTION

The present invention provides a first ebullition cooling device for aheat generating component which device comprises a boiling unit forboiling a refrigerant contained therein with the heat generated by theheat generating component as attached to an outer surface of the unit, acondensing unit disposed above the boiling unit for condensing arefrigerant vapor flowing thereinto from the boiling unit by heatexchange with an external fluid, and a communication pipeinterconnecting the boiling unit and the condensing unit and having arefrigerant vapor channel and a refrigerant condensate channel therein.

The communication pipe is thus provided between the boiling unit and thecondensing unit. For example, even if the heat receiving position in theelectronic device is a large distance away from the heat dissipatingposition of the device, or even in the case where the space betweenthese two positions is greatly limited, the cooling device is theneasily made usable in such an electronic device by adjusting the lengthof the communication pipe or the position where the pipe is to beinstalled, hence a high degree of freedom of design. The communicationpipe has a refrigerant vapor channel and a refrigerant condensatechannel therein, and there is no, interference between the refrigerantvapor and the refrigerant condensate within the communication pipe, sothat the maximum amount of heat transport remains undecreased to ensurehigh heat dissipating performance. Further even when the heat receivingposition is at a large distance from the heat dissipating position, anincrease in the amount of refrigerant to be enclosed can be prevented byminimizing the required height of the boiling unit and increasing thelength of the communication pipe.

Although the cooling device is useful when having at least onecommunication pipe, at least two communication pipes may be used. In thecase where the cooling device has at least two communication pipes, therefrigerant vapor and the refrigerant condensate are passed through therespective pipes for each pipe to pass the vapor or condensate at alower flow rate to corresponding reduce the resistance to the flow ofthe vapor or condensate and achieve improved heat dissipationperformance. Further in the case where the cooling device has at leasttwo communication pipes, the refrigerant vapor can be delivered to thecondensing unit more smoothly, permitting the refrigerant vapor to flowthrough the condensing unit uniformly.

To prevent an increase in the amount of refrigerant to be enclosed inthe device, it is desirable that the sum of the cross sectional area ofthe refrigerant vapor channel and that of the refrigerant condensatechannel of all the communication pipes be made smaller than thehorizontal cross sectional area of the interior of the boiling unit.

In the first ebullition cooling device of the invention, it is desiredthat the communication pipe comprise a pipe internally divided into twochannels of different cross sectional areas by a partition wallextending longitudinally of the pipe, the channel of greater crosssectional area in the pipe serving as the refrigerant vapor channel, thechannel of smaller cross sectional area in the pipe serving as therefrigerant condensate channel.

The pipe for constituting the communication pipe can be, for example, anextruded tube or electric resistance welded tube of aluminum or copper,and is therefore easy to make. Since the channel of greater crosssectional area in the pipe serves as the refrigerant vapor channel, andthe channel of smaller cross sectional area in the pipe as therefrigerant condensate channel, reduced resistance is offered to theflow of the refrigerant vapor or condensate through each channel,permitting the fluid to flow smoothly through the channel.

In order to avoid interference in the first heat generating componentebullition cooling device of the invention between the refrigerant vaporflowing out of an upper-end outlet of the refrigerant vapor channel intoa bottom portion of the condensing unit and a refrigerant condensateflowing out of the bottom portion of the condensing unit into anupper-end inlet of the refrigerant condensate channel, the communicationpipe preferably has an upper end portion projecting into the condensingunit bottom portion and partially cut out so that the upper-end outletof the refrigerant vapor channel is positioned above the upper-end inletof the refrigerant condensate channel.

This construction effectively obviates the interference between therefrigerant vapor flowing out of the upper-end outlet of the vaporchannel into the condensing unit bottom portion and the refrigerantcondensate flowing out of the bottom portion into the upper-end inlet ofthe condensate channel, enabling the cooling device to exhibit stillimproved heat dissipating performance.

The present invention further provides a second ebullition coolingdevice for a heat generating component which device comprises a boilingunit for boiling a refrigerant contained therein with the heat generatedby the heat generating component as attached to an outer surface of theunit, a condensing unit disposed above the boiling unit for condensing arefrigerant vapor flowing thereinto from the boiling unit by heatexchange with an external fluid, and a communication pipeinterconnecting the boiling unit and the condensing unit and provided inan inner peripheral surface thereof with a plurality of groovesextending longitudinally of the pipe and so sized as to permit arefrigerant condensate to flow down the grooves under the action ofsurface tension, the communication pipe permitting the refrigerant vaporto flow through an inside portion thereof inwardly of the grooves.

For example, even if the heat receiving position in the electronicdevice is a large distance away from the heat dissipating position ofthe device, or even in the case where the space between these twopositions is greatly limited, the ebullition cooling device described isthen easily made usable in such an electronic device by adjusting thelength of the communication pipe or the position where the pipe is to beinstalled, hence a high degree of freedom of design. The pipe forconstituting the communication pipe can be an extruded tube or electricresistance welded tube of aluminum or copper, and is therefore easy tomake. The refrigerant condensate flows down the grooves under the actionof surface tension, while the refrigerant vapor flows through an insideportion of the pipe inwardly of the grooves, i.e., through the pipecentral portion of large cross sectional area and reduced flowresistance. This assures smooth circulation of the refrigerant toeffectively obviate the flooding phenomenon. Further even when the heatreceiving position is at a large distance from the heat dissipatingposition, an increase in the amount of refrigerant to be enclosed can beprevented by minimizing the required height of the boiling unit andincreasing the length of the communication pipe.

Although the second cooling device is also useful when having at leastone communication pipe, at least two communication pipes may be used.When the cooling device has at least two communication pipes, the sameadvantages as in the case of the first cooling device are available.

Further with the second ebullition cooling device, it is also desirablethat the sum of cross sectional areas of the channels in all thecommunication pipes be smaller than the horizontal cross sectional areaof the boiling unit in order to prevent an increase in the amount ofrefrigerant to be enclosed.

The present invention provides a third ebullition cooling device for aheat generating component which device comprises a boiling unit forboiling a refrigerant contained therein with the heat generated by theheat generating component as attached to an outer surface of the unit, acondensing unit disposed above the boiling unit for condensing arefrigerant vapor flowing thereinto from the boiling unit by heatexchange with an external fluid, and a first and a second communicationpipe interconnecting the boiling unit and the condensing unit, the firstcommunication pipe having a refrigerant vapor channel therein, thesecond communication pipe having a refrigerant condensate channeltherein.

Thus, the first and second communication pipes are provided between theboiling unit and the condensing unit. For example, even if the heatreceiving position in the electronic device is a large distance awayfrom the heat dissipating position of the device, or even in the casewhere the space between these two positions is greatly limited, thecooling device described is then easily made usable in such anelectronic device by adjusting the length of the communication pipes orthe position where the pipes are to be installed, hence a high degree offreedom of design. The refrigerant vapor flows through the vapor channelwithin the first communication channel, while the refrigerant condensateflows through the condensate channel within the second communicationchannel. This eliminates the interference between the vapor and thecondensate, and diminishes the resistance to the flows of the vapor andthe condensate, consequently ensuring high heat dissipation performance.Further even when the heat receiving position is at a large distancefrom the heat dissipating position, an increase in the amount ofrefrigerant to be enclosed can be prevented by minimizing the requiredheight of the boiling unit and increasing the length of thecommunication pipes.

Further with the third ebullition cooling device, it is also desirableto prevent the increase in the amount of refrigerant to be enclosed bymaking the sum of cross sectional areas of the channels in all thecommunication pipes smaller than the horizontal cross sectional area ofthe boiling unit.

In the third ebullition cooling device of the invention, it is desirablethat two kinds of pipes be different in interior cross sectional area,one of the pipes of greater interior cross sectional area providing thefirst communication pipe and having an interior serving as therefrigerant vapor channel, the other pipe of smaller interior crosssectional area providing the second communication pipe and having aninterior serving as the refrigerant condensate channel.

The two kinds of pipes providing the first and second communicationpipes can each be, for example, an extruded pipe or electric resistancewelded pipes of aluminum or copper, and are very easy to make. The pipeof greater cross sectional area has an interior serving as therefrigerant vapor channel, and the pipe of smaller cross sectional areahas an interior serving as the refrigerant condensate channel.Accordingly, the resistance to the flow of the vapor or condensatethrough each pipe is small, permitting the fluid to flow through thepipe smoothly.

With the first to third ebullition cooling devices of the invention, theboiling unit is preferably provided on an inner surface portion thereofopposed to the heat generating component with fine surfaceirregularities for promoting boiling and heat transfer. Such finesurface irregularities are formed, for example, by applying a powder tothe inner surface portion of the boiling unit opposed to the heatgenerating component by brazing or thermal spraying, or by sintering ormachining the inner surface portion.

The fine surface irregularities formed on the boiling unit inner surfaceportion opposed to the heat generating component, i.e., on the boilingheat transfer surface, afford an increased heat transfer area, effectpromoted removal of nucleating bubbles and consequently result inpromoted boiling and heat transfer for the cooling device to exhibitgreatly improved heat dissipating performance.

With the first to third ebullition cooling devices of the invention, theheat generating component may be attached to a lower surface of a bottomportion of the boiling unit.

Since the upper surface of the boiling unit bottom portion serves as theboiling heat transfer surface in this case, the amount of refrigerant tobe enclosed can be minimized. Incidentally, the heat generatingcomponent need not always be attached to the lower surface of theboiling unit bottom portion but may be attached to other portion, e.g.,to the outer side surface of the boiling unit.

With the first to third ebullition cooling devices of the invention, theboiling unit comprises, for example, a peripheral wall of circular orrectangular or square cross section, a bottom wall closing a lower-endopening of the peripheral wall and a top wall closing an upper-endopening of the peripheral wall. The lower end of the communication pipeor the lower ends of the first and second communication pipes are joinedusually to an upper end portion of the top wall.

In the case of the ebullition cooling device of the invention for heatgenerating components, the amount of refrigerant to be enclosed can bereduced without making the boiling unit thin. The side wall of theboiling unit can therefore be given an increased thickness. Thisobviates the likelihood that even if the heat generating component isattached to the side wall outer surface, increased contact thermalresistance between the side wall and the component due to insufficientrigidity of the side wall will result in lower heat dissipatingperformance.

In the first to third ebullition cooling devices of the invention, thecondensing unit comprises an upper and a lower header tank arrangedhorizontally at a spacing, a plurality of heat exchanger tubes arrangedin parallel laterally and each having a lower end joined to the lowerheader tank and an upper end joined to the upper header tank, and heatradiating fins fixedly arranged between the adjacent heat exchange tubesand on the outer sides of heat exchange tubes at left and right ends.The upper end of the communication pipe or the upper ends of the firstand second communication pipes are joined usually to a bottom portion ofthe lower header tank.

Alternatively, the condensing unit may comprise a header tank disposedhorizontally, a plurality of heat exchanger tubes arranged in parallellaterally and each having a lower end joined to the header tank and aclosed upper end, and heat radiating fins fixedly arranged between theadjacent heat exchange tubes and on the outer sides of heat exchangetubes at left and right ends. The upper end of the communication pipe orthe upper ends of the first and second communication pipes are joinedusually to a bottom portion of the header tank.

The condensing unit may further comprise a cooling fan attached to oneof front and rear sides of the heat exchange tubes directly or by aduct. An air stream then produced by the fan efficiently dissipates heatfrom the surfaces of the heat exchange tubes and the radiating fins.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view partly broken away and showing a first embodimentof the invention, i.e., an ebullition cooling device for a heatgenerating component.

FIG. 2 includes side elevations of the ebullition cooling device, (a)showing a cooling fan as attached directly to heat exchange tubes, (b)showing the cooling fan as attached to the heat exchange tubes by aduct.

FIG. 3 includes perspective views each showing a boiling unit and partof a communication pipe on an enlarged scale, (a), (b) showing thesecomponents of different shapes.

FIG. 4( a) is an enlarged view in vertical section of the upper endportion of the communication pipe and a lower header tank of acondensing unit, and FIG. 4( b) is an enlarged view in horizontalsection of the communication pipe.

FIG. 5 shows a second embodiment of the invention, (a) being an enlargedview in vertical section of the upper end portion of a communicationpipe and a lower header tank of a condensing unit, (b) being an enlargedview in horizontal section of the communication pipe.

FIG. 6 is a graph showing the relationships between the quantity of heatgenerated by a component and the thermal resistance, as established inExamples 1, 2 and Comparative Example.

BEST MODE OF CARRYING OUT THE INVENTION

FIGS. 1 to 4 show a first embodiment of the invention, i.e., anebullition cooling device 1 for a heat generating component. Theillustrated cooling device 1 comprises a boiling unit 2 for boiling arefrigerant A contained therein with the heat generated by a heatgenerating component B attached to an outer surface of the unit, acondensing unit 3 for condensing a refrigerant vapor A1 flowingthereinto from the boiling unit 2 by heat exchange with an externalfluid C, and a communication pipe 4 interconnecting the units 2 and 3and having a refrigerant vapor channel 41 and a refrigerant condensatechannel 42 therein. The sum of the cross sectional area of the vaporchannel 41 and that of the condensate channel 42 is smaller than thehorizontal cross sectional area of the interior of the boiling unit 2.

With reference to FIGS. 1 to 3, the boiling unit 2 comprises aperipheral wall 21, a bottom wall 22 closing a lower-end opening of theperipheral wall 21 and a top wall 23 closing an upper-end opening of thewall 21. The walls 21, 22 and 23 constituting the unit 2 are made of ametal material such as aluminum or copper. The peripheral wall 21 may becircular in cross section as shown in FIG. 3( a), or rectangular orsquare in cross section as seen in FIG. 3( b). The peripheral wall 21 ismade, for example, by machining a block material or prepared from anextrudate. When the block material is used for machining, the bottomwall 22 or the top wall 23 is usually formed integrally with theperipheral wall 21. The bottom wall 22 and the top wall 23 are made, forexample, from a plate material or extrudate and joined respectively tothe lower and upper ends of the peripheral wall 21 as by welding orbrazing.

The boiling unit 2 has contained therein a fluorocarbon refrigerant A(see FIG. 1). The refrigerant A may alternatively be a naturalrefrigerant such as water.

As shown in FIGS. 1 and 2, the heat generating component B, such as adiode or transistor, is attached to the lower surface of the bottom wall22.

As shown in FIG. 1, the inner surface portion of the boiling unit 2opposed to the heat generating component B, i.e., the upper surface ofthe bottom wall 22, has fine surface irregularities 24 for promotingboiling and heat transfer. These fine surface irregularities 24 areformed, for example, by brazing an aluminum powder to the upper surfaceof the bottom wall 22. Alternatively, the fine surface irregularities 24can be formed by sintering the upper surface of the bottom wall 22,applying a power to the upper surface of the bottom wall 22 by thermalspraying, or etching, knurling, sandblasting or otherwise machining theupper surface of the bottom wall 22. The fine surface irregularities 24give an increased heat transfer area and promote removal of nucleatingbubbles to ensure greatly improved heat dissipating performance.

With reference to FIGS. 1 and 2, the condensing unit 3 comprises anupper and a lower header tank 31 arranged horizontally at a spacing, aplurality of heat exchanger tubes 32 arranged in parallel laterally andeach having a lower end joined to the lower header tank 31 and an upperend joined to the upper header tank 31, heat radiating fins 33 fixedlyarranged between the adjacent heat exchange tubes 32 and on the outersides of heat exchange tubes 32 at the left and right ends, and acooling fan 34 attached to the heat exchange tubes 32 on the rear sidethereof. The upper and lower header tanks 31, heat exchange tubes 32 andheat radiating fins 33 are made of a metal material such as aluminum orcopper.

The upper and lower header tanks 31 each comprise an extruded tube orelectric resistance welded tube having a circular cross section andclosed with an end plate at each of opposite end openings thereof.

The heat exchange tubes 32 each comprise a flat extruded tube orelectric resistance welded tube having a rectangular or oblong crosssection and having upper and lower ends inserted respectively in theupper and lower header tanks 31 and joined thereto as by brazing orwelding.

Each heat radiating fin 33 is in the form of a corrugated fin and joinedto the outer surface of the heat exchange tube 32 as by brazing orwelding. The corrugated fin may be a louver fin or offset fin for use inmotor vehicle air conditioners, or a plane fin having a large number ofperforations, i.e., a perforated fin, in addition to a usual plane fin.Use of such a fin leads to a still higher heat dissipation effect.

The heat radiating fin 33 positioned at each of the left and right endsis provided on the outer side thereof with a side plate 36 comprising ametal plate, such as an aluminum plate or copper plate, and joined tothe fin as by brazing or welding.

The cooling fan 34 to be used is, for example, an axial-flow fan. Thecooling fan 34 may be attached directly to the heat exchange tubes 32 onthe rear side thereof as shown in FIG. 2( a), or attached to the heatexchange tubes 32 on the rear side thereof by a duct 35 as shown in FIG.2( b). The cooling fan 34 may be in a suction arrangement wherein theair intake side thereof faces toward the heat exchange tubes 32(forward) as shown in FIG. 2, or conversely in a forcing-in arrangementwherein the air discharge side of the fan faces toward the heat exchangetubes 32 (forward). Of course, the cooling fan 34 may be attached to theheat exchange tubes 32 at the front side thereof. When operated, thecooling fan 34 causes air C serving as an external fluid to flow betweenthe heat exchange tubes 32 from the front rearward to effect heatexchange with the refrigerant vapor A1 flowing through the heat exchangetube. The external fluid to be subjected to heat exchange with therefrigerant vapor A1 can be selected from among known cooling fluidssuch as water, besides air C mentioned above. The construction of thecondensing unit 3 is suitably modified in accordance with the fluid tobe used in this case.

The communication pipe 4 comprises a pipe internally divided into twochannels 41, 42 of different cross sectional areas by a partition wall43 extending longitudinally of the pipe as shown in FIG. 4. The channelof greater cross sectional area in the pipe serves as the refrigerantvapor channel 41, and the channel of smaller cross sectional area as therefrigerant condensate channel 42. The pipe providing the communicationpipe 4 comprises an extruded tube or electric resistance welded tube ofaluminum or copper. The communication pipe 4 may be circular in crosssection as seen in FIG. 4 or FIG. 3( a) or rectangular or square incross section as seen in FIG. 3( b).

In order to avoid interference between the refrigerant vapor A1 flowingout of an upper-end outlet 411 of the refrigerant vapor channel 41 intothe lower header tank 31 (the bottom portion of the condensing unit 3)and the refrigerant condensate A2 flowing out of the lower header tank31 into an upper-end inlet 421 of the refrigerant condensate channel 42,the communication pipe 4 has an upper end portion projecting into thelower header tank 31 and partially cut out so that the upper-end outlet411 of the vapor channel 41 is positioned above the upper-end inlet 421of the condensate channel 42 as shown in FIG. 4( a). Thus, the upper-endoutlet 411 of the vapor channel 41 is positioned at a level higher thanthe liquid level of the refrigerant condensate A2 in the lower headertank 31, and the upper-end inlet 421 of the condensate channel 42 ispositioned at a level lower than the liquid level of the refrigerantcondensate A2 in the lower header tank 31. In the lower header tank 31,therefore, the flow of refrigerant vapor A1 acting to rise due tobuoyancy is completely separated from the flow of refrigerant condensateA2 acting to flow down under gravity. This ensures smooth circulation ofthe refrigerant A to obtain a sufficient maximum amount of heattransport.

The length of the communication pipe 4 can be altered suitably inaccordance with the distance between the heat receiving position (i.e.,the location where the heat generating component B is installed) withinthe electronic device and the heat radiating position (i.e., in thevicinity of the location where a vent is provided). Further in the caseof the ebullition cooling device of FIG. 1, the upper end of thecommunication pipe 4 is joined to the lengthwise midportion of the lowerheader tank 31 of the condensing unit 3, and the lower end of thecommunication pipe 4 is jointed to the center portion of the top wall 23of the boiling unit 2, whereas when there is a need to dispose othercomponent, device or the like in a portion of the interior space of theelectronic device between the boiling unit 2 and the condensing unit 3,the communication pipe 4 may be shifted so as to avoid interference withthe component.

A description will now be given of the principle of operation of theebullition cooling device 1 described. When a large quantity of heat isgenerated by the component B during the operation of the electronicdevice, the heat is transferred to the boiling unit 2 to boil therefrigerant A within this unit 2. At this time, upper surface of thebottom wall 22, i.e., the fine surface irregularities 24 provided on theboiling heat transfer surface, promote boiling and heat transfer,assuring efficient heat transfer from the heat generating component B tothe refrigerant A. The refrigerant vapor A1 released from the boilingrefrigerant A ascends the vapor channel 41 of the communication pipe 4from the boiling unit 2, flows out of the upper-end outlet 411 of thechannel 41 into the lower header tank 31 of the condensing unit 3, fromwhich the vapor dividedly flows upward through the heat exchange tubes32. While flowing through the tubes 32, the refrigerant vapor A1 issubjected, through the tubes 32 and fins 33, to heat exchange with theair C caused to flow between the heat exchange tubes 32 from the frontrearward by the cooling fan 34 and condenses on the inner surfaces ofthe tubes 32. The refrigerant vapor A1 failing to undergo condensationwithin the tubes 32 flows into the upper header tank 31 and thereafterflows into some of the tubes 32 again for condensation. The refrigerantcondensate A2 produced within the tubes 32 flows down the tubes 32 undergravity and then flows into the lower header tank 31, in which thecondensate stays temporarily and then flows into the condensate channel42 of the communication pipe 4 from the upper-end inlet 421. Since theoutlet 411 of the vapor channel 41 is positioned above this inlet 421 ofthe condensate channel 42, the low of the condensate A2 is completelyseparated from the flow of refrigerant vapor A1. This ensures smoothcirculation of the refrigerant A. The condensate A2 is returned via thecondensate channel 42 to the boiling unit 2, where the liquid is boiledagain. The refrigerant A repeatedly undergoes the phase changesdescribed above, whereby the heat generating component B is continuouslycooled.

FIG. 5 shows a second embodiment of the invention. The ebullitioncooling device according to this embodiment for use with a heatgenerating component comprises a boiling unit 2, a condensing unit 3,and a communication pipe 40 interconnecting the boiling unit 2 and thecondensing unit 3. The communication pipe 40 comprises a pipe providedin the inner peripheral surface thereof with a plurality of grooves 401extending longitudinally of the pipe and so sized as to permit arefrigerant condensate A2 to flow down the grooves under the action ofsurface tension. The communication pipe 40 permits a refrigerant vaporA1 to flow through an inside portion 402 thereof inwardly of the grooves401. The interior of the pipe 40 has a cross sectional area smaller thanthe horizontal cross sectional area of the interior of the boiling unit2. The pipe providing the communication pipe 40 is in the form of anextruded tube or electric resistance welded tube of aluminum or copper.The pipe 40 has an upper end portion projecting into a lower header tank31 so that an upper-end opening of the pipe 40 will be positioned at thesame level as, or at a level slightly lower than, the liquid level ofthe refrigerant condensate A2 in the lower header tank 31. When theebullition cooling device has the communication pipe 40 described above,the refrigerant condensate A2 flows down the grooves 401 of the pipe 40under the action of surface tension, while the refrigerant vapor A1flows through the inside portion 402 of the pipe 40 inwardly of thegrooves 401, i.e., through the pipe central portion 402 of large crosssectional area and reduced flow resistance. This assures smoothcirculation of the refrigerant to effectively obviate the floodingphenomenon. The second embodiment is otherwise substantially the same asthe first.

In Example 1, an ebullition cooling device was fabricated which had thesame construction as shown in FIGS. 1 to 4. The boiling unit was madefrom aluminum, and fine surface irregularities were formed on the uppersurface of its bottom wall for promoting boiling and heat transfer, bybrazing an aluminum powder to the surface. The upper and lower headertanks, heat exchange tubes and heat radiating fins of the condensingunit 3 were also made from aluminum. Used as the communication pipe wasan extruded tube of aluminum having a partition wall in its interior anda circular cross section. A fluorocarbon was used as the refrigerant tobe enclosed in the device.

Fabricated in Example 2 was an ebullition cooling device having the sameconstruction as the device of Example 1 except that the bottom wall ofthe boiling unit had a flat upper surface having no fine surfaceirregularities for promoting boiling and heat transfer.

Further fabricated in Comparative Example was an ebullition coolingdevice having the same construction as the device of Example 1 exceptthat the communication pipe used was an extruded aluminum tube having acircular cross section and having no partition wall in its interior.

An electric heater serving as a substitute for a heat generatingcomponent was attached to the lower surface of boiling unit bottom wallof each cooling device to measure the thermal resistance R (K/W) of thecooling device at varying quantities of heat generation Q (W) of theheater. FIG. 6 shows the results of measurement. FIG. 6 indicates thatExample 1 achieved very high cooling performance in terms of thermalresistance of 0.15 K/W at a heat generation quantity of 300 W. Example 2resulted in a thermal resistance of 0.26 K/W at a heat generationquantity of 200 W, whereas a markedly increased thermal resistance wasmeasured with a further increase in heat generation quantity. A floodingphenomenon occurred within the communication pipe of the device ofComparative Example, so that a markedly increased thermal resistance wasmeasured at a heat generation quantity of about 30 W. The devicetherefore failed to produce a satisfactory cooling effect.

1. An ebullition cooling device for a heat generating component,comprising: a boiling unit configured to boil a refrigerant containedtherein with a heat generated by the heat generating component attachedto an outer surface of the boiling unit; a condensing unit disposedabove the boiling unit and configured to condense a refrigerant vaporflowing thereinto from the boiling unit by heat exchange with anexternal fluid; and a communication pipe interconnecting the boilingunit and the condensing unit and having a refrigerant vapor channel anda refrigerant condensate channel therein, the communication pipe havingan upper end portion projecting into a bottom portion of the condensingunit, the upper end portion having a partially cutout portion so that anupper-end outlet of the refrigerant vapor channel is positioned above anupper-end inlet of the refrigerant condensate channel so as to avoidinterference between a refrigerant vapor flowing out of the upper-endoutlet of the refrigerant vapor channel into the bottom portion of thecondensing unit and a refrigerant condensate flowing out of the bottomportion of the condensing unit into the upper-end inlet of therefrigerant condensate channel.
 2. A heat generating componentebullition cooling device according to claim 1 wherein the communicationpipe comprises a pipe internally divided into two channels of differentcross sectional areas by a partition wall extending longitudinally ofthe pipe, the channel of greater cross sectional area in the pipeserving as the refrigerant vapor channel, the channel of smaller crosssectional area in the pipe serving as the refrigerant condensatechannel.
 3. A heat generating component ebullition cooling deviceaccording to claim 1 wherein the boiling unit is provided on an innersurface portion thereof opposed to the heat generating component withfine surface irregularities for promoting boiling and heat transfer. 4.A heat generating component ebullition cooling device according to claim3 wherein the fine surface irregularities are formed by applying apowder to the inner surface portion of the boiling unit opposed to theheat generating component by brazing or thermal spraying, or bysintering or machining the inner surface portion.
 5. A heat generatingcomponent ebullition cooling device according to claim 1 wherein theheat generating component is attached to a lower surface of a bottomportion of the boiling unit.
 6. A heat generating component ebullitioncooling device according to claim 1 wherein the boiling unit comprises aperipheral wall of circular or rectangular or square cross section, abottom wall closing a lower-end opening of the peripheral wall and a topwall closing an upper-end opening of the peripheral wall.
 7. A heatgenerating component ebullition cooling device according to claim 1wherein the condensing unit comprises an upper and a lower header tankarranged horizontally at a spacing, a plurality of heat exchanger tubesarranged in parallel laterally and each having a lower end joined to thelower header tank and an upper end joined to the upper header tank, andheat radiating fins fixedly arranged between the adjacent heat exchangetubes and on the outer sides of heat exchange tubes at left and rightends.
 8. A heat generating component ebullition cooling device accordingto claim 7 wherein the condensing unit further comprises a cooling fanattached to one of front and rear sides of the heat exchange tubesdirectly or by a duct.
 9. A heat generating component ebullition coolingdevice according to claim 1 wherein the condensing unit comprises aheader tank disposed horizontally, a plurality of heat exchanger tubesarranged in parallel laterally and each having a lower end joined to theheader tank and a closed upper end, and heat radiating fins fixedlyarranged between the adjacent heat exchange tubes and on the outer sidesof heat exchange tubes at left and right ends.
 10. A heat generatingcomponent ebullition cooling device according to claim 1 wherein therefrigerant vapor channel and the refrigerant condensate channel of thecommunication pipe are positioned at a center portion of a top wall ofthe boiling unit.
 11. A heat generating component ebullition coolingdevice according to claim 1 wherein a sum of a cross sectional area ofthe refrigerant vapor channel and a cross sectional area of therefrigerant condensate channel is smaller than a cross sectional area ofan interior of the boiling unit.
 12. An ebullition cooling devicecomprising: a boiling unit configured to boil a refrigerant contained inthe boiling unit with a heat generated by a heat generating componentattached to an outer surface of the boiling unit; a condensing unitdisposed above the boiling unit and configured to condense a refrigerantvapor flowing in the condensing unit from the boiling unit by heatexchange with an external fluid; and a communication pipeinterconnecting the boiling unit and the condensing unit, thecommunication pipe being joined to a center portion of a top wall of theboiling unit, the communication pipe being internally divided into arefrigerant vapor channel and a refrigerant condensate channel, thecommunication pipe including an upper end portion projecting into abottom portion of the condensing unit, the upper end portion having anupper-end outlet of the refrigerant vapor channel and an upper-end inletof the refrigerant condensate channel, the upper-end outlet of therefrigerant vapor channel being positioned above the upper-end inlet ofthe refrigerant condensate channel.